Centrifugal pumps have been used for many years to move or pump fluids, usually liquids or a liquid-gas mixture. For instance, scavenging pumps are widely used in hydromechanical power systems for scavenging liquids, such as oil, used for lubricating and/or cooling components of the system.
However, scavenging of a fluid which has a high air-to-oil ratio mixture (e.g. 4:1 and up) continues to be a problem for such positive displacement pumps because of the carry-over of air from the discharge to the inlet and from leakage. Centrifugal pumps work quite well for pumping substantially 100% liquid, such as oil. But an impeller which is designed to pump oil generally will not pump air or pump a high air-to-oil mixture efficiently.
The problem arises from cavitation created behind the rotating impeller blades. In high speed centrifugal pumps, this commonly is called supercavitation. In other words, a centrifugal pump normally includes a housing defining a pumping chamber within which an impeller structure rotates. The chamber generally is cylindrical or of a contour complementary to the contour of the impeller blades. A fluid inlet is provided to the chamber at a radially inward location in relation to a radially outward location for an outlet from the chamber through which the fluid is pumped. As the impeller structure rotates at a high speed, cavitation occurs behind the impeller blades. In essence, cavitation is caused by vapor cavities formed on the trailing side of the blades as they rotate at high speed. The cavities generally are at the vapor pressure of the liquid being pumped. This phenomena is akin to an object, such as a projectile, passing through water at a high rate of speed. A vapor cavity is formed behind the projectile trailing its direction of movement.
Normally, cavitation is undesirable in centrifugal pumps because the vapor cavities create a power loss, cause "pitting" damage to the surrounding chamber walls and cause backflow or carry-over of the air from the discharge to the inlet of the pump. Therefore, attempts heretofore have been made to diminish the vapor cavities by separating the air from the pumped liquid. This is accomplished by providing bleed passages from the pumping chamber to "drain" the cavities which usually form near the roots of the impeller blades because the air is lighter than the pumped liquid.
This invention is directed to a novel approach of taking advantage of the cavitation phenomenon and actually using the inherent low pressure of the vapor cavities to enhance pumping of fluids having a high air-to-liquid ratio and even fluids which may be substantially gas.